Integrated circuits (ILC) utilize micro-components that require electrical energy. Neither the micron sized conductors nor the micro-components are 100 percent efficient. Both convert some of the electrical energy used in their computations into heat. In the early versions of these integrated circuits having relatively few components per unit area, natural convection cooling proved adequate to limit the operating temperatures to safe values. As technology allowed packing more components into an integrated package the heat generated required motor driven fans mounted directly on the IC packages, thereby providing forced convection cooling, to control the package temperature. The manufacturers have even provided finned surface extenders to be mounted to the IC packages with a heat conducting paste to better dissipate the IC package heat to the fan forced air stream. All of these heat dissipation schemes have employed macro-cooling methods to cool micro components.
Significant increases in component density and accompanying heat dissipation rates have acted to raise operating temperatures of the IC packages to such levels that their operating life can be endangered and in the alternative to limit the heat dissipation rates, thereby limiting the ultimate capability of the IC package.
The current invention is directed to micro means for sharply improving the coefficients of heat transfer between the coolant and the IC and for providing improved means for removing heat generated by an IC. The proposed micro pump and heat exchanger allows present high density ICs to operate at lower temperature, thereby providing longer life. The present invention, by providing sharply improved flow and heat transfer over the heat dissipation area further has the capability of allowing future ICs to be manufactured with higher component densities and to operate at higher heat dissipation levels without exceeding life threatening component temperatures.
This system offers several features including; 1) applying the electric field directly to the heat transfer surface using MEMS (Micro Electronic Mechanical Systems, technology to provide ultra thin liquid films; 2) providing the required pumping action to bring the working fluid to the heat transfer surface; and 3) increasing the effective heat transfer coefficient at the heat transfer surface by thin-film evaporation. Each electrode typically has a thickness of 0.3 xcexcm to 10 xcexcm and a width of 2 xcexcm to 50 xcexcm. The gap between the electrodes depends on the design and application and may vary over the range from 2 xcexcm to 100 xcexcm.
A typical fabrication sequence is described below. However, it is expected that more modern and rapid manufacturing sequences will be developed or applied to the process to secure the desired arrangement of the electrodes. Therefore, it is emphasized that the novelty of the invention lies in the use of the micro-electrode arrangement to achieve polarization primping of the cooling fluid and the application of an electric field through the micro electrodes to improve the heat transfer coefficients over the heat transfer area.
Typical fabrication begins with wafer or substrate pre-metalization cleaning. The substrate is typically quartz but sapphire or other similar material may be employed. After cleansing, 300 xc3x85 thickness Chromium and 2500 xc3x85 thickness Platinum (1xc3x85=0.0001 xcexcm) is deposited using an e-beam evaporator. A 1.5 xcexcm thick layer of photo resist is applied over the deposited metals followed by a soft bake at 100xc2x0 C. Photolithography is employed to create the desired electrode pattern followed by a hard-bake at 120xc2x0 C. While Ion beam-milling was employed, a variety of other etching techniques such as wet etchinq and deep reactive ion etching are available.
The Cr/Pt film is etched to give the heater and electrode patterns. Following the micro fabrication, the packaging is performed.
A preferred cooling fluid suitable for use in this invention that is highly subject to electrical polarization is a mixture of about 50 percent each of nonafluorolsobutylether and nonafluorobutylether offered by 3M Company located in St. Paul Minn. (1 800 364-3577) under the trade name HFE-7100 (dielectric constant k=7.4). This fluid has a typical boiling point at atmospheric pressure of 60C (xcx9c140F) and a viscosity of 0.23 CPS at 23C (73.4F). Among other useable fluids are those which have low electrical conductivity and dielectric constants in the range of 2 to 100. Examples of these are deionized (DI) water (k=78.5), HFC-134a (k=9.5), L-13791 (k=7.39) and methoxy nonafluorobutane (C4F9OCH3).
The use of the electrohydrodynamic technique for micro-scale fluid pumping has been investigated by a number of researchers over the past half decade (Bart et al., 1990; Richter et al., 1991; Fuhr et al., 1992; Fuhr et al., 1994; Cho and Kim, 1995; and Ahn and Kim, 1997).
Bart discloses an EHD pumping principle employing a traveling electrical wave or charge imposed between electrodes positioned in a substantially parallel array whereby a non-electrically conducting fluid is moved transverse to the electrodes by a sinusoidally applied voltage. Bart points out that his principle works only if the electrodes are freely suspended within the fluid to be pumped and will not work if the electrodes are positioned against the surface to be cooled.
Richter et al. (1991) demonstrated a micro-machined ion-drag EHD pump consisting of pairs of facing permeable or perforated substantially planar grids through which the pumped fluid moves. Richter displays an array of pairs for increasing the pumping head. Richter points out (p. 160 col. 1) that the polarization or xe2x80x98dielectrophoreticxe2x80x99 force xe2x80x9ccannot lead to a permanent fluid motion for DC fields . . . xe2x80x9d Further, none of Richter""s grids are in direct contact with any surface to be cooled.
Fuhr (1992, employs a arid of micro electrodes applied to a surface but teaches a single phase or poly-phase electrical alternating potential applied to his electrodes. Further, Fuhr""s pumped fluid moves transversely to the electrodes.
Fuhr (1994) again teaches a traveling wave pumping design and suggests that a square wave format is superior to sinusoidal wave format. He further points out that traveling wave pumping principles require that the fluid pumped exhibit a gradient in the properties of electrical conductivity or permittivity, a characteristic not required by the present invention.
Chol (1995) teaches flow direction that is transverse to the electrode direction and the use of six phase AC as the driving potential.
Ahn (1997) teaches an ion-drag principle where the fluid flow is transverse the linear direction of the micro-electrodes.
The present invention is based on a polarization pumping principle. No previous work was found that addressed the use of EHD pumping based on polarization principles.
The invention discloses a micro pump for moving a cooling fluid over a heated surface to be cooled. The pump comprises an array of substantially parallel linear micro electrodes positioned on the hot surface. A conduit is provided enclosing the array and positioned to cause flow parallel to the direction of the electrodes. The conduit has an interior periphery including the hot electrode bearing surface. The electrodes are electrically connected in at least two groups and a voltage source is employed for applying a non reversing electromotive force between the electrode groups.
An object of the invention is to provide low cost, easily applied means for circulating, without moving parts, a cooling fluid in heat transfer relation to a small surface requiring cooling.
A further object is to provide such means employing micro-electrodes that can be applied to the surface itself.
A further object is to provide such means that utilize fluid polarization principles.
A further object is to provide such means that require unusually small amounts of electrical power.
A further object is to provide such means that require only direct current energization and do not require single or multi-phase alternating currents for electrode energization.
A further object is to provide such circulating or pumping means for a fluid that evaporates on contact with the surface being cooled.
A further object is to provide such circulating means that includes means for applying an electric field directly to the surface being cooled. thereby improving the heat transfer coefficient between the cooling fluid and the surface.
A further object is to provide an active thin film evaporation and cooling process.
A further object is to deploy the pumping means over the cooled surface and over an adjacent surface and where the means for applying the electric field to the cooled surface is an extension of the micro-electrodes that comprise the pump.
A further object is to provide such circulating means to a surface positioned at an angle to the horizontal and especially where the fluid moves from a lower position on the surface to a higher position.
A further object is to position the cooled surface at a right angle to the horizontal.
A further object is to employ a closed circulating system for the fluid circulated.
A still further object is to employ a volatile liquid as the fluid circulated and to deploy an externally cooled condenser to condense vapor generated at the cooled surface to the liquid state for reuse at the cooled surface.
A further object is to provide xe2x80x98gravityxe2x80x99 circulating means for returning the condensed vapor to the surface.
A further object is to employ a second pump for facilitating the return of liquid from the condenser to the cooled surface.
A further object is to employ the principle of micro-electro-mechanical systems or MEMS to achieve the above objects.
Other equally important objects and objectives will be noted as the detailed exposition of the construction and usage of the invention is perused in the text below.
FIG. 1 shows a side elevation of a plump and heat exchanger of the invention in heat transfer relationship to a heat producing integrated circuit package.
FIG. 2 is cross-sectional view 2 of device of FIG. 1 illustrating the gross internal electrode layout of the pump-heat exchanger of the invention.
FIG. 3 is cross section 3 of the device of FIG. 1 showing an end view of the pump-heat exchanger of FIG. 1.
FIG. 4 Illustrates, in gross, the electrode positioning within the flow channel.
FIG. 5 shows a pump-heat exchanger of the invention combined with a heat producing integrated circuit package.
FIG. 6 is a plan view of the pump-heat exchange assembly showing the hidden electrodes
FIG. 7 is a highly enlarged cross section of the electrodes and their enclosure including typical electrode spacings and dimensions.
FIG. 8 shows the angular limits of effective performance of the assembly.
FIG. 9 and 10 show details of variations in electrode shape and spacing at the pump inlet and outlet.
FIG. 11 shows one version of a potential cooling circuit employing a secondary external pump.